5 Water Quality

5.1 Introduction

5.1.1 This section deals with the assessment of the impacts on water quality of the construction and operation of the proposed Peng Chau Sewage Treatment Works Upgrade. Baseline information including the existing water quality, hydrographic conditions of the Study area, projected flow and pollution loads of the sewage treatment works are presented and the potential impacts are assessed with reference to the relevant environmental requirements.

5.1.2 The near field model Visjet is employed to predict the initial dilution of sewage plume during the operational phase of Peng Chau STW upgrade. No far-field hydrodynamic modelling is required as the effluent discharged from the proposed STW upgrade would be small.

5.2 Relevant Legislation, Policies, Plans, Standards and Criteria

Water Pollution Control Ordinance

5.2.1 The principal legislation governing water quality in Hong Kong is the Water Pollution Control Ordinance (WPCO) (Cap 358). Under an amendment to the original Ordinance of 1980, the Territorial Waters of Hong Kong waters have been subdivided into ten Water Control Zones (WCZs) with each WCZ being assigned a designated set of statutory Water Quality Objectives (WQOs). These WQOs relate to the Beneficial Uses (BU) and assimilative capacity of the particular water body or part thereof. The Tai Lei Island of Peng Chau falls into the Southern WCZ (SWCZ). The relevant WQOs are given in Table 5-1.

Table 5-1 Selection of Water Quality Objectives – Southern WCZ

Parameters	Beneficial Uses	Criterion
Suspended Solids	Marine Waters	Waste discharges shall neither cause the natural ambient level to be raised by more than 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities.
Dissolved Oxygen	Marine Waters excepting Fish Culture Subzones	Waste discharges shall not cause the level of dissolved oxygen to fall below 4 mg/L for 90% of the sampling occasions during the whole year, value should be calculated as the water column average.
	Fish Culture Subzones	Depth-averaged DO shall not be less than 5mg/L for 90% of the samples collected in the year. In addition, the dissolved oxygen concentration should not be less than 2mg/L within bottom 2m of seabed for 90% of the sampling occasions during the whole year.
	Inland waters of the Zone	Waste dischargers shall not cause the level of dissolved oxygen to be less than 4 mg/L.
	Cooling Water	The dissolved oxygen concentration at the cooling water intake point should not be less than 2mg/L.
Temperature	Whole Zone	The discharge should not raise the ambient water temperature by 2°C.
PH	Marine Water except Bathing Beach Subzone	The pH of water should be within the range of 6.5-8.5 units. Change due to waste discharge should not exceed 0.2.
	Bathing Beach Subzone	pH should be within 6.0 to 9.0 units for 95% of samples. Waste discharge shall not cause the natural pH range to extend by more than 0.5 units.
Salinity	Whole Zone	Change due to waste discharge not to exceed 10% of natural ambient level.
E. coli	Secondary Contact Recreation Subzones and Fish Culture Subzone	Annual geometric mean should not exceed 610cfu/100mL.
E. coli	Bathing Beach Subzone	E. coli level should not exceed 180 cfu/100mL, calculated as geometric mean of all samples collected from March to October inclusive in one calendar year.
Un-ionised ammonia	Whole Zone	Annual mean not to exceed 0.021mg/L.
Nutrients	Marine Waters	Annual depth average inorganic nitrogen not to exceed 0.1mg/L.
Toxicants	Whole Zone	Not to be present at levels producing significant toxic effect.

Technical Memorandum for Effluents Discharged into drainage and Sewerage Systems Inland and Coastal Waters

5.2.2 The WPCO also stipulates that discharge of any waste and polluting matters into Hong Kong waters including sewers and surface drains are subject to the control of discharge licences. Any discharges and effluent should comply with the Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (EPD, 1991), referred to as the Technical Memorandum on Effluent Standards (TMES). The effluent discharge water quality standards specified for the inshore waters of Southern WCZ are shown in Table 5-2.

Table 5-2 Water Quality Standards for Effluent Discharges into the Inshore Waters of Southern Water Control Zone

Flow Rate (m₃/day)	<10	>10 and <400	>200 and <400	>400 and <600	>600 and <800	>800 and <1000	>1000 and <1500	>1500 and <2000	>2000 and <3000	>3000 and <4000	>4000 and <5000	>5000 and <6000
Determinant
pH (pH units)	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9	6-9
Temperature (^oC)	40	40	40	40	40	40	40	40	40	40	40	40
Colour (lovibond units)--25m cell length	1	1	1	1	1	1	1	1	1	1	1	1
Suspended solids	50	30	30	30	30	30	30	30	30	30	30	30
BOD	50	20	20	20	20	20	20	20	20	20	20	20
COD	100	80	80	80	80	80	80	80	80	80	80	80
Oil & Grease	30	20	20	20	20	20	20	20	20	20	20	10
Iron	15	10	10	7	5	4	3	2	1	1	0.8	0.6
Boron	5	4	3	2	2	1.5	1.1	0.8	0.5	0.4	0.3	0.2
Barium	5	4	3	2	2	1.5	1.1	0.8	0.5	0.4	0.3	0.2
Mercury	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Other toxic metals individually	1	1	0.8	0.7	0.5	0.4	0.3	0.2	0.15	0.1	0.1	0.1
Total toxic metals	2	2	1.6	1.4	1	0.8	0.6	0.4	0.3	0.2	0.1	0.1
Cyanide	0.2	0.1	0.1	0.1	0.1	0.1	0.05	0.05	0.03	0.02	0.02	0.01
Phenols	0.5	0.5	0.5	0.3	0.25	0.2	0.1	0.1	0.1	0.1	0.1	0.1
Sulphide	5	5	5	5	5	5	2.5	2.5	1.5	1	1	0.5
Total residual chlorine	1	1	1	1	1	1	1	1	1	1	1	1
Total nitrogen	100	100	80	80	80	80	50	50	50	50	50	30
Total phosphorus	10	10	8	8	8	8	5	5	5	5	5	5
Surfactants (total)	20	15	15	15	15	15	10	10	10	10	10	10
E. coli (count/100 ml)	1000	1000	1000	1000	1000	1000	1000	1000	1000	1000	1000	1000

5.2.3 In order to minimise impacts on the receiving waters, construction site drainage will be managed following the recommended handling and disposal guidelines as detailed in the EPD’s Practice Note for Professional Persons on Construction Site Drainage (ProPECC PN 1/94). Relevant guidelines are also stipulated in Annexes 6 and 14 of the Technical Memorandum of EIA Process.

Effluent Discharge Criteria Set by Outlying Island Sewerage Master Plan and Project Study Brief

5.2.4 The OISMP Stage 2 Review requires the Peng Chau STW Upgrade to meet a more stringent total nitrogen discharge concentration as compared with Table 5-2. The annual average total nitrogen is less or equal to 10mg/L. It was also recommended that the STW design should aim for ability to achieve an average effluent quality of less or equal to 5mg/L ultimately. These requirements are further specified in the Project Study Brief and are included in the water quality assessment of this Project.

5.3 EXISTING MARINE environment

Surrounding Marine Environment and Conditions

5.3.1 Both Tai Lei and Peng Chau Islands are located to the east of Lantau Island. The assessment area of water quality is specified as 1km of the proposed works and a 2km area is specified for cumulative impact. There are several beaches scattered on these islands, including Discovery Bay, Nim Shue Wan, Peng Chau Tung Wan and Silver Mine Bay. Secondary contact recreation subzones are located on both sides of the channel and the subzone on the west covers the whole Discovery Bay. There are no WSD seawater intake or mariculture zones in the area. Live and dead coral communities are also found around Tai Lei Island and mostly in the shallow channel area to the east of Tai Lei Island.

5.3.2 The marine conditions of Peng Chau, as described in the SMP 2002 are:

(a) The surrounding marine waters are remote from the major mass flows and are relatively poorly flushed;

(b) Depth of water surrounding Peng Chau are generally in the range 5-7m, and the channel immediately to the west of Tai Lei Island is over 10m deep;

(c) The average current through the strait between Tai Lei and Lantau Island is about 0.2 m/s hence the average mass flow is about 1,500 m³/s; and

(d) There is no obvious and systematic residual but between 10 and 20% of the water passing through the straits on any six hour tide does not return.

5.3.3 The Peng Chau islands and Tai Lei are located within the beneficial use of secondary contact recreation area (Figure 5-1).

Baseline Conditions

5.3.4 The marine waters of the Southern WCZ had been routinely monitored by EPD on a bi-monthly basis until Year 2000, when the monitoring frequency changed to once per month. The closest EPD monitoring stations are SM 10 and SM 11 (Figure 5-1). These two stations are located relatively inshore, with water depth ranges from approximately 7m to 10m. Data sets of relevant water quality parameters of these two stations from 1997 to 2001 have been reviewed to establish the existing ambient conditions.

5.3.5 EPD also undertakes sediment analyses in the Southern WCZ. The closest sediment monitoring station to Peng Chau and Tai Lei Island is SS5 (Figure 5-1). The sediment quality from 1997 to 2001 has been summarised and presented in Marine Water Quality in Hong Kong in 2001 (EPD, 2002c).

5.3.6 The hydrodynamics, seawater and sediment quality of Peng Chau and Tai Lei have been investigated and presented in the EIA report on Outlying Islands Sewerage –Stage I Phase I (Maunsell, 1997). The survey locations are shown in Figure 5-2. Wet and dry season surveys were carried out in September and October 1996, respectively.

5.3.7 In addition, sediment sampling and analysis was carried out to the southwest of Tai Lei Island, under this Project. Detail findings of this sediment quality results are presented in Section 6.

5.3.8 The EPD water quality and sediment quality monitoring data, as well as the survey results presented in Maunsell 1997 reports are summarised and discussed in the following sections.

Marine Water Quality

5.3.9 Table 5-3 presents the summary statistics of key water quality parameters from the EPD monitoring stations SM 10 and SM 11.

5.3.10 Review of historical summary statistics indicated the following:

(a) Salinity – The annual average salinity values are fairly constant and are in the range of 29 to 30psu. Monthly range, however, indicates that salinity varies seasonally.

(b) Temperature – The temperature of marine water varies seasonally. The depth average and range water temperature for the two stations through the reviewed monitoring period appeared similar.

(c) Dissolved Oxygen (DO) – Although the annual mean depth averaged DO levels at Stations SM 10 and 11 ranged between 5.9-6.96 mg/L, indicating full compliance with the relevant WQO of 4mg/L, the EPD marine water quality report 1998 states that there were DO non-compliance at Station SM 10. Further review on EPD’s raw water quality data revealed that non-compliance only occurred once out of six data sets. This non-compliance may be considered as an episodic event, as the rest data show consistently high DO values in other monitoring years.

(d) The range of bottom DO level is 5.8-6.76 mg/L and is in full compliance with the WQO for bottom water. The relatively high DO levels indicate the area is oxygenated.

(e) pH – The average pH value obtained from both stations are within the WQO range of 6.5-8.5 units.

(f) Total Inorganic Nitrogen (TIN) – Water quality exhibited high nutrient levels with TIN level generally measured in excess of the WQO of 0.1 mg/L. The annual depth-averaged TIN levels ranged between 0.23 to 0.33 mg/L.

(g) Unionised Ammonia – The annual depth –averaged unionised ammonia ranged from 0.002 to 0.004 mg/L, which are well below the WQO of 0.021mg/L.

(h) Ammonium Nitrogen – The annual average of ammonium nitrogen concentration ranged from 0.06 to 0.09mg/L. Similar ranges are seen in both stations.

Table 5-3 Summary Statistics of Water Quality at Stations SM 10 and SM 11 (Selected Parameters)

WQ Parameters

1997

1998

1999

2000

2001

SM 10

SM11

SM 10

SM 11

SM 10

SM 11

SM 10

SM 11

SM 10

SM 11

Salinity (psu)

29.3

(20.8-30.2)

29.4

(20.6-32.9)

30.2

(26.9-33.4)

30.8

(28.2-33.6)

30.2

(25.3-32.5)

30.3

(25.4-32.7)

29.9

(22.6-32.9)

30.2

(25-32.9)

29.5

(23.2-32.2)

29.3

(22.1-32.2)

Temperature (°C)

22.7

(16.3 –27.9)

23.4

(18.9 –27.9)

23.4

(16.3-27.2)

23.2

(16.1-27.0)

23.6

(17.3 –27.3)

23.5

(17.5 – 27.2)

23.0

(16.6 –27.6)

23.0

(16.3-27.6)

23.8

(16.8 –28.3)

23.9

(16.5-28.9)

DO mg/L

6.96

(6.32 – 7.90)

6.96

(5.98 – 8.07)

5.9

(3.2 –7.2)

6.0

(4.0-7.5)

6.1

(4.7-7.7)

6.5

(5.0-9.3)

6.5

(4.9 –8.0)

6.5

(5.0 –8.2)

6.0

(4.7 –7.4)

6.3

(4.5 – 8.4)

DO (bottom) mg/L

6.76

(6.27 – 8.00)

6.01

(4.06 – 6.99)

5.9

(3.9-7.2)

5.8

(4.0-7.3)

6.3

(4.9-8.0)

6.5

(4.9-8.9)

6.5

(4.7 –8.0)

6.4

(4.8 –8.6)

6.2

(4.8 –7.5)

6.1

(4.6 –8.8)

DO Saturation %

95.6

(86.1-111.1)

96.9

(81.4-115.6)

(70-101)

(87-110)

(67-97)

(70-118)

(72 –105)

(74 –106)

(65-108)

(68-125)

DO Saturation (Bottom) %

92.8

(85.9 – 99.5)

83.8

(59.8 – 92.8)

(83-98)

(86-93)

(69-101)

(70-112)

(68 –104)

(71 –106)

(70 –106)

(66-130)

7.8

(6.7 – 8.2)

8.0

(7.7 –8.4)

7.9

(7.5-8.1)

7.9

(7.5-8.2)

8.0

(7.8-8.2)

8.0

(7.8-8.3)

8.0

(7.6 –8.3)

8.0

(7.7 –8.3)

8.2

(7.7-8.7)

8.2

(7.8 –8.8)

Total Inorganic Nitrogen (mg/L)

0.28

(0.15- 0.48)

0.26

(0.09 –0.46)

0.27

(0.14-0.43)

0.25

(0.16-0.36)

0.25

(0.11-0.4)

0.23

(0.08-0.38)

0.3

(0.09 –0.59)

0.26

(0.07 –0.47)

0.33

(0.21 –0.44)

0.29

(0.17 –0.41)

Unionised Ammonia (mg/L)

0.002

(<0.001-0.009)

0.003

(0.001-0.006)

0.003

(0.001-0.009)

0.003

(0.001-0.01)

0.003

(0.001-0.005)

0.003

(0.001-0.006)

0.004

(0.001-0.014)

0.004

(0.001-0.013)

0.004

(<0.001-0.01)

0.004

(<0.001-0.01)

Ammonium-N (mg/L)

0.08

(0.02-0.14)

0.08

(0.03-0.14)

0.09

(0.02-0.15)

0.09

(0.03-0.14)

0.07

(0.06-0.1)

0.06

(0.04-0.11)

0.09

(0.05-0.23)

0.09

(0.04-0.16)

0.09

(0.01-0.23)

0.07

(0.01-0.16)

Total Kjeldahl Nitrogen

0.75

(0.37-1.2)

0.75

(0.39-1.2)

0.78

(0.46-1.25)

0.78

(0.44-1.1)

0.3

(0.14-0.43)

0.2

(0.12-0.36)

0.25

(0.18-0.35)

0.24

(0.17-0.33)

0.24

(0.17-0.34)

0.21

(0.1-0.3)

5-day BOD (mg/L)

1.2

(0.6-3.1)

0.9

(0.5-1.8)

0.9

(0.1-1.8)

0.9

(0.1-1.8)

0.6

(0.2-1.0)

0.7

(0.3-1.2)

0.8

(0.4-2.4)

1.0

(0.4-2.1)

0.9

(0.1-2.6)

1.1

(0.1-3.5)

Suspended Solids (mg/L)

6.4

(2.4 – 17)

6.0

(2.2 – 14.0)

6.9

(4.1-9.9)

8.9

(4.9-14.7)

12.5

(4.3-45.0)

6.4

(2.5-11.3)

12.8

(1.7 –36.0)

8.7

(1.9-30.0)

17.9

(6.4-50.5)

10.9

(3.4 –28.7)

E. coli (cfu / 100ml)

(1 – 310)

(1- 300)

(2-300)

(1-36)

(1-450)

(1-120)

(1 –190)

(1-100)

(1-85)

(1-59)

Note: 1. Except as specified, data presented are depth-averaged data.

2. Data presented are annual arithmetic means except for E.coli which are geometric means.

3. Data enclosed in brackets indicate the ranges.

Source: The 1997 data are derived from raw data provided by EPD/WPG

Data for 1998 to 2001 are from Marine Water Quality in Hong Kong in 1998, 1999, 2000 and 2001 (EPD)

(i) Total Kjeldahl Nitrogen (TKN) – A decrease trend is observed for this parameter from 0.8mg/L to about 0.25mg/L and remains relatively constant for the last few years.

(j) 5 Day Biochemical Oxygen Demand (BOD) – The 5-day BOD concentration remains low for the entire monitoring period.

(k) Suspended Solids (SS) – The annual depth –averaged SS level was between 6.0 to 17.9 mg/L. The fluctuation in SS level throughout the year may be due to the natural variation in the ambient water body.

(l) E. coli – The geometric mean E. coli levels for the area ranged from 4 to 58 cfu/100ml, which was within the WQO standard for secondary contact recreation zone.

5.3.11 In summary, EPD data indicate that marine water surrounding Peng Chau and Tai Lei generally exhibits good quality except TIN, for which the baseline level has already exceeded the WQO level. The elevated baseline TIN may be due to the discharge from Pearl River Estuary.

5.3.12 Monitoring results of selected parameters of marine water quality from the Manusell 1997 Studies are given in Tables 5-4 and 5-5.

Table 5-4 Wet Season Water Quality Baseline Monitoring Results – Peng Chau (1996)

Parameters (mg/L unless otherwise stated)	Station 2	Station 3	Station 5	Station 7	Station 8
Temperature (°C)	27.7	27.7	27.7	27.7	27.7
Salinity (ppt)	29.8	29.7	29.6	29.3	29.8
Dissolved Oxygen	5.4	5.4	5.4	5.5	5.3
Dissolved Oxygen saturation %	80	81	81	82	80
pH (pH units)	7.8	7.8	7.7	7.8	7.8
Suspended Solids	13.3	12.9	16.3	12.1	15.1
Ammonium Nitrogen	0.184	0.177	0.173	0.163	0.198
Total Inorganic Nitrogen	0.337	0.333	0.347	0.341	0.355
Total Kjeldahl Nitrogen	0.25	0.242	0.242	0.283	0.233
5-day Biochemical Oxygen Demand	<1	<1	<1	<1	<1
E. coli*	132	24	48	21	85

* No unit is reported in Maunsell 1997 report.

Notes: All values are depth and tidally averaged for a neap tide and a spring tide

Table 5-5 Dry Season Water Quality Baseline Monitoring Results – Peng Chau (1996)

Parameters (mg/L unless otherwise stated) *	Station 2	Station 3	Station 5	Station 7	Station 8
Temperature (°C)	22.9	22.6	22.5	22.6	22.9
Salinity (ppt)	33.6	33.6	33.5	33.6	34.2
Dissolved Oxygen	6.5	6.5	6.7	6.9	6.6
Dissolved Oxygen saturation %	92	91	93	97	94
pH (pH units)	8.1	8.1	8.1	8.1	8.1
Suspended Solids	17.2	14.5	18.2	18.5	14.2
Ammonium Nitrogen	0.047	0.028	0.043	0.023	0.033
Total Inorganic Nitrogen	0.161	0.159	0.195	0.153	0.148
Total Kjeldahl Nitrogen	0.483	0.4	0.417	0.383	0.433
5-day Biochemical Oxygen Demand	1.8	1.8	2.5	2.5	2.8
E. coli*	<1	2	40	3	16

No unit is reported in Maunsell 1997 report.

Notes: All values are depth and tidally averaged for a neap tide

5.3.13 Seasonal trends are seen in the above tables. The wet season survey results show a general lower salinity, DO content, Percent DO saturation, BOD and organic nitrogen (represented by TKN). In the dry season, ammonium nitrogen, total inorganic nitrogen and bacteria counts are lower. All the 1996 survey results are in the range of EPD historical monitoring data at SM 10 and SM11, suggesting data from these two monitoring stations can be utilised as baseline water quality data for Peng Chau and Tai Lei marine conditions.

Stratification Conditions

5.3.14 Pearl River Estuary discharge also affects the salinity of Hong Kong marine environment. Although fresh water enters Hong Kong waters all year around, relatively large volume flushes in during the wet season, causing the upper seawater to be less dense than the bottom seawater.

5.3.15 Analyses of historical water quality data recorded by EPD at stations SM 10 and SM 11 have been carried out to obtain density profiles in the marine waters surrounding Peng Chau and Tai Lei Islands. Figure 5-3 shows the average bi-monthly density profiles from 1995 to 2000 at both stations. It is seen from historical density profiles that wet season is represented by June and August, when distinct density differences are observed throughout the water column. The wet season density ranges from 14.5 to 19 sigma t units, with maximum density differences from top to bottom approximately 3.5 sigma t units. Density profiles in other months exhibit constant density throughout the water column. Density ranges from 20 to 24 sigma t units.

Current Velocity and Direction

5.3.16 Hydrodynamic surveys carried out in 1996 (Maunsell, 1997) covered the north, the west and the south of Tai Lei Island, and included the area in the channel between Tai Lei Island and Discovery Bay. Thewater depth of theselocations are generally over 6m. Figure 5-4and Figure 5-5 depict the frequency distribution of current velocity at Peng Chau for wet and dry seasons, respectively.

5.3.17 Figure 5-4 indicates that current speed was below 0.4m/s for 90% of the surveying period at wet season. In 10% of the time the speed was less than 0.05m/s. The dominant current were in the directions of north/northeast and south/southwest. Current speed in dry season surveying period is similar to wet season survey. The dominant current directions, however, were north and south.

5.3.18 Current vectors were analysed for both wet and dry seasons. Measurement of two ebb tides and two flood tides were carried out. During wet season, the direction of current vector suggested that during ebb tide, seawater moved predominantly from north/northeast to south the along the channel to the west Tai Lei Island. During flood tide, where majority of water move in the opposite direction along the channel to the west, movement of water along the shallow channel between Tai Lei was also anticipated (Figure 5-6aandFigure 5-6b). Similar observations were also found in dry season.

Marine Sediment Quality

5.3.19 The sediment quality at EPD monitoring station SS5 was classified in accordance with the Environment, Transport and Works Bureau (ETWB) Technical Circular (Works) No. 34/2002 Management of Dredged /Excavated Sediment. The ETWB Technical Circular set forth the Lower Chemical Exceedance Level (LCEL) and Upper Chemical Exceedance Level (UCEL) values for contaminant including heavy metals, metalloid, organic-PAHs, organic-non-PAHs and organometallics.

5.3.20 The summary figures in Marine Water Quality in Hong Kong 2001 indicate that sediment quality at station SS5 is at or below the LCEL level for heavy metals, metalloid, organic-PAHs and organic-non-PAHs (Table 5-6). The contaminant level of the sediment belongs to Category L, indicating low level of contamination. No data is shown for organometallics as it is only required for area of specific industrial discharge.

Table 5-6 Summary of Sediment Quality in Station SS5 (1997-2001)

Contaminant	LCEL	UCEL	SS5
*Metals (mg/kg dry weight)*
Cadmium (Cd)	1.5	4	£1.5
Chromium (Cr)	80	160	£80
Copper (Cu)	65	110	£65
Mercury (Hg)	0.5	1	£0.5
Nickel (Ni)	40	40	£40
Lead (Pb)	75	110	£75
Silver (Ag)	1	2	£1
Zinc (Zn)	200	270	£200
*Metalloid (mg/kg dry weight)*
Arsenic (As)	12	42	£12
*Organic –PAHs (ug/kg/dry weight)*
Low Molecular Weight PAHs	550	3160	£550
High Molecular Weight PAHs	1700	9600	£1700
*Organic-non-PAHs (ug/kg/dry weight)*
Total PCBs	23	180	£23

5.3.21 Measurement of sediment quality was also carried out across the channel between Tai Lei Island and Discovery Bay. The results were classified according to EPD Technical Circular No. TC 1-1-92 “Classification of Dredged Sediments for Marine Disposal”. For easy comparison between these surveyed data and the data presented in Marine Water Quality in Hong Kong (EPD, 2001), the following table (Table 5-7) includes the chemical exceedance level and the survey results.

Table 5-7 Sediment Quality around Peng Chau (1996)

Contaminant	LCEL	UCEL	PC1	PC2	PC3	PC4
*Metals (mg/kg dry weight)*
Cadmium (Cd)	1.5	4	0.05	0.09	0.05	0.14
Chromium (Cr)	80	160	30	32.5	13.8	17.8
Copper (Cu)	65	110	29.3	42.6	17.8	24.9
Mercury (Hg)	0.5	1	0.14	0.2	0.11	0.12
Nickel (Ni)	40	40	17.3	19.5	8.7	10.9
Lead (Pb)	75	110	38	48.7	30.4	57.5
Silver (Ag)	1	2	NA	NA	NA	NA
Zinc (Zn)	200	270	85	104	51	85
*Metalloid (mg/kg dry weight)*
Arsenic (As)	12	42	NA	NA	NA	NA
*Organic –PAHs (ug/kg/dry weight)*
Low Molecular Weight PAHs	550	3160	NA	NA	NA	NA
High Molecular Weight PAHs	1700	9600	NA	NA	NA	NA
*Organic-non-PAHs (ug/kg/dry weight)*
Total PCBs	23	180	NA	NA	NA	NA

NA – Not applicable as no measurement was performed

5.3.22 Table 5-7 shows that the selected measured sediment parameters complied with the LCEL level. The sediment contamination level for heavy metals was low. As no major change of waste discharge pattern is anticipated for the intervening years, it is suggested that the sediment quality contamination level in the vicinity of Peng Chau and Tai Lei Island still remains low.

5.3.23 According to the findings in the sediment sampling and analyses conducted under the Project, the sediment quality to the southwest of Tai Lei Island was determined to be Category L.

5.4 Marine Water Sensitive Receivers

5.4.1 Existing or potential beneficial uses in the vicinity of Peng Chau and Tai Lei Island that are sensitive to water pollution within 1 kilometer of the proposed outfall locations are illustrated in Figure 5-7 and provided in Table 5-8. Table 5-8 provides the estimated distance from the proposed outfalls to identified marine water sensitive receivers.

Table 5-8 Identified Marine Water Sensitive Receivers

Beneficial Uses	Name	Distance from Outfall (Diffuser zone)
Water Impact Assessment Zone
Marina	Discovery Bay Marina Facility (Hai Tei Wan)	556m
Beach (Non -gazetted)	Peng Chau Tung Wan	870m
Secondary contact recreation subzones	Both sides of the channel between Tai Lei and Lantau Island	Immediate surrounding
Marine Habitat	Northeast of Tai Lei Island: corals of low abundance but with relatively high species richness Southwest of Tai Lei Island: corals of low abundance (small isolated patches)	253m northeast approx. 100 west and southwest

5.5 Design capacity and effluent requirement of Peng Chau STW upgrade

5.5.1 The aim of Peng Chau Sewage Treatment Works Upgrade is to reconstruct the existing Peng Chau STW as a secondary treatment works incorporating nitrification, denitrification, and disinfection as well as other facilities including storm tanks to meet the future development with projected Peng Chau population of 6,200. The calculated average dry weather flows (ADWF) is 1,580m³/day and the peak dry weather flow (PDWF) is calculated to be 4,740 m³/day (3x ADWF). The influent characteristics and the required effluent concentrations are provided in Table 5-9a.

Table 5-9a Design Parameters and Capacities of Peng Chau STW Upgrade

Design Parameters	Planned Scenario
Population	6,200
Average Dry Weather Flows (m³/day)	1,580
Peaking Factor (excluding stormwater allowance)	3
Peak Flow (m³/day)	4,740
Influent Concentration ⁽¹⁾(mg/L)
§ BOD § TSS § TKN § Ammonium-N § E. coli (cfu/100ml)	202 182 39 23 1.9x10⁷
Discharge Requirement (mg/L)	Minimum Standard
§ BOD (95 percentile) § SS (95 percentile) § TN (Annual Average)/ Ammonium-N (95 percentile) § Total Residual Chlorine (95 percentile) § E. coli (cfu/100ml) (Monthly Geometric Mean)	20 30 10 1 ⁽²⁾ 1000

Note: (1) Loads concentration for both average dry weather flows and peak flows

(2) According to the assessment results in Section 5.7, it is recommended that the maximum allowable TRC concentration should not be more than 0.8 mg/L (Section 5.7.27 refers)

5.5.2 It is specified in the Project requirement that the proposed Peng Chau STW Upgrade should allow provision in the plant layout for the necessary upgrading works to meet the following alternative effluent standard in future (Table 5-9b). Figures 2-4a and 2-4b also show the provisional layout (additional SBR unit) for achieving more stringent TIN requirement.

Table 5-9b Alternative Effluent Standards

Discharge Requirement (mg/L)

Minimum Standard

§ BOD (95 percentile)

§ SS (95 percentile)

§ TN (Annual Average)

§ Total Residual Chlorine (95 percentile)

§ E. coli (cfu/100ml) (Monthly Geometric Mean)

1 ⁽¹⁾

1000

Note: (1) According to the assessment results in Section 5.7, it is recommended that the maximum allowable TRC concentration should not be more than 0.8 mg/L (Section 5.7.27 refers)

5.5.3 The evaluation of operational water quality impact would be based on discharge requirement stipulated in Table 5-9a. An assessment on TN of 5mg/L would also be carried out.

5.6 Impact Identification

Construction Phase

Upgrade of Peng Chau Sewage Treatment Works

5.6.1 The proposed project involves the following construction works:

i. Phase 1 Works:

(a) construction of a new STW adjacent to the existing STW comprising secondary treatment with nitrification, de-nitrification and disinfection;

(b) construction of two new submarine outfalls;

(d) provision of associated sludge treatment facilities;

(e) extension of inlet pumping mains; and

(f) construction of equalisation tank.

ii. Phase 2 Works:

(a) demolition of the existing treatment facilities;

(b) construction of sludge drying bed; and

5.6.2 The potential construction water quality impacts can be categorised into land-based impacts and marine based impacts.

5.6.3 A major source of terrestrial water quality impact will be run-off from stormwater, and site construction activities associated with upgrading works. Minimal water quality impact is anticipated for laying of inlet pumping mains. Pollutants in the runoff would mainly contain suspended solids from excavation or dredging and/or oil and grease from mechanical equipment operation. Such runoff may pollute the receiving waters when entering the marine environment.

5.6.4 Most of the terrestrial construction activities are confined within the existing Peng Chau sewage treatment works boundary. Runoff water are likely to be collected and settled, and returned to the inlet of the sewage treatment works as much as practicable for further settling and treatment. Domestic sewage would also be collected and return to the STW. No interruption of sewage treatment works would be anticipated during the upgrade construction of Peng Chau STW. To ensure that adequate treatment capacities can be provided, new facilities would be built prior to demolition of the old facilities. Operation of STW would be guaranteed at all times during construction.

5.6.5 The anticipated impact from the construction of terrestrial components would be insignificant. Nevertheless, for good site practice and appropriate mitigation measures are recommended to be implemented during the construction of new facilities and demolition of old facilities.

Submarine Outfall and Emergency Overflow Outfall

5.6.6 The construction of submarine outfall and emergency overflow outfall would involve using open trench method. Grab dredgers would be used to excavate seabed to form a trench. Subsequent construction activities include trench trimming, in -situ fabrication, in-situ testing, outfall pipeline laying and backfilling.

5.6.7 The proposed trench dimensions for both the submarine outfall and emergency overflow are shown in Figure 5-8. The estimated dredged volume is about 22,000 m³. Dredging and removing of this amount of marine mud is considered to be small scale. Typically grab dredge of estimated dredging rate about 365m³/hr would be used. With this dredging rate, the anticipated dredging duration would be approximately 2 weeks.

5.6.8 Building of connection shafts between the terrestrial effluent pipe and outfall pipe would require destruction of the existing seawall. The destruction and re-construction of seawall, however, would not have adverse environmental impact. Environmental impacts would arise from the formation of sediment plumes from dredging. The dispersion of the sediment plume would elevate the suspended solid concentration in the vicinity of the dredged area. Elevated suspended solids would then be carried by tidal currents and dispersed to nearby sensitive receivers including the coral colonies found at the outcrop boulder to the southwest of Tai Lei Island. Evaluation of dredging effect is presented in later sections.

Reclamation Works of Peng Chau Helipad

5.6.9 The construction of Peng Chau Helipad would start at about the same time as the construction of the Peng Chau STW Upgrade project. The dredging and reclamation works of Helipad would have potential impacts to the identified marine sensitive receivers under the STW Upgrade project. However, the marine works of the proposed Peng Chau STW Upgrade has been scheduled after the completion of Helipad reclamation. Thus cumulative construction water quality impacts on the identified marine sensitive receivers would not be anticipated.

Operational Phase

Peng Chau STW Upgrade

5.6.10 The Peng Chau STW Upgrade aims at improving overall marine water quality of Peng Chau. Raw sewage from Peng Chau households would be collected, treated and discharged via the proposed submarine outfall. After secondary treatment and nitrogen removal process, the pollution loading entering into the marine environment would be significantly less as compared with the existing conditions, where secondary treatment is only provided for residents in Kam Peng Estate and Peng Lai Court and sewage from the remaining households of Peng Chau is discharged into sea without much treatment. The impact from the proposed Project is considered to be positive.

5.6.11 The positive impact, however, can not be quantified as there is no available information on the untreated discharge. The environmental impacts identified during operational phase are the chemical species of concern which exists in the sewage effluent.

Nitrogen Species

5.6.12 The main concern of the discharge would be the total nitrogen content as the baseline concentration in the receiving waters has already exceeded the WQO limits. The unionised ammonia level in the total nitrogen would be toxic to marine organism when present at high level. The WQO limit of unionised ammonia is 0.021mg/L.

Chlorination and Dechlorination

5.6.13 Chlorination will be used in Peng Chau STW Upgrade for disinfection purpose. Subsequent dechlorination will be provided to lower the concentration of total residual chlorine in the effluent. In view of low design flow of the Peng Chau STW Upgrade, a desktop assessment involving literature review and initial dilution modelling is conducted for assessing the impacts of Chlorination By-Products (CBP) and Total Residual Chlorine (TRC). No field investigation has been conducted. Such approach is considered appropriate taken into consideration the relatively small scale; hence the environmental impacts, of the project.

Chlorination By-Products

5.6.14 The formation of chlorination by-products (CBPs) is a complex process controlled by numerous parameters. In simple terms, in the process of destruction of pathogenic and other harmful organisms by chlorination, certain organic constituents in wastewater interfere with chlorine to form CBPs. Common classes include trihalomethane (THM); haloacetic acids (HAA); halogenated phenols; and haloacetonitriles. Chlorinated effluents may contain these groups of CBPs but generally at very low concentrations. For organic CBP classes, the chlorination yield (i.e. the amount of organic-CBP formed during the chlorination process) is approximately 1% of chlorine dosage for treated effluent after secondary treatment (EPD, 2000b). In view of the fact that secondary treatment and nitrification/denitrification processes will be adopted for Peng Chau STW Upgrade, organic pollutant and ammonia/nitrate levels in the treated effluent will be low. Hence, formation of organic CBPs are not expected to be high. Based on the current treatment process design of this Project, the chlorine dosage is about 10mg/L and the concentration of organic-CBP classes is estimated to be about 0.1mg/L. After initial dilution, the concentration will be insignificant, and hence its environmental impacts will be negligible because of the very small quantity of effluent to be discharged.

5.6.15 Another dominant group of CBP is chloramines (as ingredient of TRC), which is the product formed when ammonia/nitrate react with chlorinated effluent. With the target treatment level of this Project, the concentration of chloramines will be less than 1mg/L, the design level of TRC (Table 5-9a and section 5.6.19 refer). This will be further reduced after initial dilution and dechlorination process. Detailed TRC assessment is presented in Section 5.7.

5.6.16 There have been numerous publications on the formation of chlorination by-products but limited scientific information on the aquatic life and human health impacts of CBPs in wastewater effluent discharged to the sea. Available data indicated that chloramines are more toxic to aquatic life, compared to other CBPs such as trihalomethane and haloacetic acids, with LC50 values (i.e. median lethal concentration of a toxic substance that kills 50% of test organisms under specific test conditions) of about 0.06mg/L to 1mg/L (EPD, 2000b). The same data source also indicated that the lowest CBP toxicity value is 0.22mg/L for bromoacetic acid (one group of haloacetic acid). This number, however, is considered conservative as the reported value is an EC 50 value (a concentration where 50% of the test organism would be affected) instead of an LC 50 value. It is also noted that the test organism is a fresh water species and the toxicity value may not be applicable to marine species, which are likely to be affected by Peng Chau STW effluent discharge.

5.6.17 Nevertheless, a general approach to convert the toxicity value to water quality criteria to be observed at the edge of mixing zone is to apply safety factors to the toxicity data. However, due to the limited variability of CBPs toxicity data (where only the EC50 and LC50 values are available), the toxicity values obtained are not considered as conservative as those tests which yield NOEC/LOEC/NOAEL[i] values. It is thus recommended that conservative safety factors should be used for assessment of the CBPs toxicity. A safety factor 100 is adopted based on the professional judgement, and the water quality criteria for bromoacetic acid at the edge of mixing zone would be 2.2mg/L. As indicated in Section 5.6.14 that the concentration of organic CBP classes is estimated to be 0.1mg/L, and the predicted minimum initial dilution factor of outfall discharge is 76.1 (S.5.7.21). The respective concentration of organic CBP at the edge of initial dilution zone would be 1.3mg/L. This value is lower than the bromoacetic acid water quality criteria of 2.2 mg/L and the water quality impact with respect to bromoacetic acid is thus considered insignificant.

5.6.18 The above toxicity data are only be considered as reference information due to the fact that no field investigation has been conducted particularly on the Peng Chau STW effluent. For the purpose of desktop assessment, given the organic-CBPs are lower toxicity, it is anticipated that discharge of about 0.1mg/L organic-CBP would have insignificant effect on aquatic life.

5.6.19 There are two routes where human would be exposed to CBPs in wastewater effluent. One is incidental ingestion of seawater while swimming or engaged in other on-water activities, or from accidental emersion. Another one is ingestion of locally caught seafood that may have accumulated CBPs from the water. The toxicity of CBPs to human from seawater ingestion may generally be assessed by evaluation of the standards or guidance governing the allowed concentrations in drinking water. For swimmers and others exposed via incidental ingestion, the acute, short-term health advisories are more pertinent. These advisories (i.e. National Health and Medical Research Council (NHMRC) of Australia and United States Environmental Protection Agency (USEPA)) suggested a safe human ingestion range for various CBP classes from 1 to 6 mg/L. Discharge of 0.1mg/L organic-CBP is unlikely to cause any human health impact from incidental seawater ingestion.

5.6.20 Similar to human health impact via seawater ingestion, the CBPs toxicity from ingestion of caught seafood may also be assessed by evaluation of the relevant standards/criteria. The USEPA has developed water quality criteria (National Recommended Water Quality Criteria 2002) for human health impact from consumption of organisms for priority toxic pollutants including CBP chemicals, and the ranges are from 0.013mg/L to 0.47mg/L. While the discharge is estimated to be 0.1mg/L, the concentration of CBPs at the edge of mixing zone would be governed by the initial dilution factors. In this case, a minimum initial dilution factor of 10 is adequate to safeguard the water quality at the edge of mixing zone, and such factor could be easily achieved by provision of submarine outfall. Therefore, human health impact from consumption of organisms that exposed to Peng Chau STW effluent is not anticipated to be significant. Detailed information of initial dilution predictions is provided in S5.7.20.

5.6.21 Given the small flow of effluent discharge and the provision of submarine outfall for effluent dispersion, the organic-CBP in the treated effluent should have insignificant impact to both the ecology and human health.

Total Residual Chlorine

5.6.22 Chloramines formed from chlorination process, together with the hypochlorite ion (OCl-) and the related hypochlorous acid (HOCL), are referred to as total residual chlorine (TRC). TRC in wastewater effluents is usually the main toxicant suppressing the diversity, size and quantity of fish in fresh water bodies receiving chlorinated secondary effluent (Paller et al. 1983). Chloramines levels can be reduced by dechlorination process such as adding sulphur dioxide, sodium sulphite, sodium metabisulphite or activated carbon. With the provision of dechlorination facilities, the concentration of chloramines can be further reduced to a safety margin.

5.6.23 The USEPA has established criteria for residual chlorine in seawater for the protection of aquatic life. The acute and chronic toxicity criteria at the edge of mixing zone are 0.013mg/L and 0.0075mg/L, respectively (EPD, 2000b). Assessment would be carried out based on the effluent TRC design criteria of 1mg/L as specified in Table 5-9a. Reduction of effluent TRC would be recommended if assessment results show that the toxicity criterion at the edge of initial dilution zones is not complied. Detailed assessments are provided in Section 5.7.Treatment Scheme

Emergency Overflow

5.6.24 The emergency overflow events are anticipated in case of pump failure, the electrical power supply interruption, mechanical failure of primary, secondary/ tertiary treatment units, disinfection units, as well as blockage of submarine outfall. The overflow event is expected to be in-frequent and of short duration. The evaluation of near-field impacts from effluent discharge and emergency overflow have been carried out. The subsequent far field estimation of emergency overflow is presented in later sections.

Proposed Peng Chau Typhoon Shelter

5.6.25 The implementation of Peng Chau Typhoon Shelter would further reduce the water movement in the area. Potential cumulative impacts are addressed in the later sections.

5.7 IMPACT Assessment

Construction Phase

Construction and Demolition of Land Components

5.7.1 As described in the previous sections, the water quality impact from construction and demolition of land base components of Peng Chau STW upgrade is not significant. Nevertheless, mitigation measures are recommended to be implemented for good site practice.

Submarine Outfall and Emergency Overflow

Uncertainties in Assessment Methodology

5.7.2 Quantitative uncertainties in the assessment of the impacts from suspended sediment plumes should be considered when drawing conclusions from the assessment. In carrying out the assessment, realistic worst case assumptions have been made in order to provide a conservative assessment of environmental impacts including:

o The assessment is based on previous modelling results which input the sediment lost to suspension at the water surface to minimise local settling and, therefore, would predict higher concentrations remote from the works area:

o The assessment is based on the peak dredging and filling rates, which will only occur for short periods of time; and

o The calculations of loss rates of sediment to suspension are based on conservative estimates for the types of plant and methods of working.

Assessment on Impacts of Submarine and Emergency Outfall

5.7.3 The construction of submarine and emergency outfall would involve dredging and disposal of approximately 22,000 m³ of marine mud. The cross section of the dredged area is provided in Figure 5-8.

5.7.4 For outfall construction by dredging, typically an open grab dredger with a dredging rate of; say; 365m³/hr would be used. Given the shallow marine water environment in the vicinity of Peng Chau and Tai Lei, a simple sediment plumes model proposed by Wilson (1979) is adopted to predict the downstream sediment concentration.

5.7.5 This model is considered appropriate for the calculation of suspended sediment concentration from submarine outfall dredging to the south-west of Tai Lei because the equation is based on a continuous line source of sediment, which is a reasonable approximation of the loss of sediment to suspension during grab dredging. It is appropriate for areas where the tidal current is uni-directional for each phase of the tidal cycle (i.e. the ebb and flood phases), which is the case at Peng Chau (see Figure 5-6). This method is applicable for suspended sediment plumes for length no greater than the maximum tidal excursion. At Tai Lei maximum tidal current speed may go up to 0.6m/s (see Figures 5-4 and 5-5) and a representative period for each phrase of the tidal cycle in Hong Kong is 6 hours. The tidal excursion may be calculated according to the following equation:

Tidal Excursion = maximum speed * period * 2/p

5.7.6 The tidal excursion is thus calculated to be 8.25km and hence this approach may be considered appropriate because of the low rate of dredging and thus the expected limited extent of the plumes, which will certainly be within the tidal excursion.

5.7.7 The formula for estimating the concentration of suspended solids (SS) at a certain distance from the source is:

C(x) = q/(w*x* D *Öp)

Where C(x)	=	Concentration of suspended solids at distance from the source (kg/m³)
q	=	Sediment Release Rate (kg/s)
D	=	Water Depth (m)
x	=	Distance from source (m)
w	=	Diffusion velocity (m/s)

5.7.8 Sediment loss rates have been reviewed in previous studies (ERM, 1998) for dredging activities. It was concluded that for grab dredgers working in areas within significant amounts of debris on the seabed, the sediment loss rate would be 25kg/m³. For areas where debris was unlikely to hinder the operations of dredging, sediment loss rate of 17kg/m³ was suggested.

5.7.9 For the impact assessment of dredging activities in the surrounding water of Tai Lei Island, the more conservative sediment loss rate of 25kg/m³ is adopted as there are two barging piers on the Island. It is expected the debris on the seabed may hinder the dredging operation.

5.7.10 The typical dredging rate of a grab dredger is 365m³/hr, giving a sediment release rate (q) of 2.53kg/s. The average water depth taken at the centre of the 100m submarine outfall is assumed to be 5m. The diffusion velocity that represents the reduction in the centre-line concentration due to lateral spreading, as proposed in Wilson’s paper is 0.01m/s. The concentration of depth averaged suspended solids with respect to different downstream distances are shown in Table 5-10.

Table 5-10 Suspended Solids Concentration at Distances from Source (Unmitigated)

Distance (m)	Predicted Suspended Solids Concentration (mg/L)	Total Suspended Solids Concentration (with background) (mg/L)
50	572.0	586.4
100	286.0	300.4
200	143.0	157.4
250	114.4	128.8
300	95.3	109.7
400	71.5	85.9
500	57.2	71.6
600	47.7	62.1
700	40.9	55.3
800	35.8	50.2
900	31.8	46.2
1000	28.6	43.0

5.7.11 The predicted allowable increase in suspended solids (SS elevation) should be less than 30% of the ambient concentration of SS at all times. The nearby EPD’s routine water quality monitoring station is SM10. However, given the proximity of SM 10 and Penny’s Bay, the water quality monitoring results in year 2001 might include the cumulative contribution from large scaled marine works in the vicinity. The monitoring results of SM 10 suspended solids concentration from 1997 to 2000 are taken as the baseline. The 90 percentile SS concentration from 1997 to 2000 of SM 10 is calculated to be 19.6mg/L and the 30% allowance is 5.9 mg/L. The predicted suspended solids concentrations, as shown in Table 5-10 exceed the WQO significantly. Although the dredging activity is short and temporary (approximately 2 weeks as estimated), the impact to the marine sensitive receivers such as coral communities found approximately 250m away from the dredging source would be adverse. Mitigation measures are required to be implemented to reduce the impact.

5.7.12 Mitigation measures recommended to minimise the water quality impact include using of closed grab dredger, reducing the dredging rate and incorporating silt curtains. A typical suspended solids reduction of 75% can be achieved with the incorporation of silt curtain. Two-To further ensure this reduction rate, layer silt curtains are recommendedhave generally been used for dredging projects of larger scale to further ensure this reduction. However, as the scale of proposed project is considered small, it is recommended to use single layer silt curtain which can achieve minimum 75% solid reduction and higher reduction performance is more preferred. (see Figure 5-9).Figure 5-9 depicts a typical silt curtain configuration.

5.7.13 Reduction in dredging rate would also decrease the dispersion of suspended solids. However, this in turn would prolong the dredging duration. It is recommended the dredging rate is reduced to 55 m³/hour. With the reduced dredging rate,dredging duration is estimated to be less than 2 months. The predicted suspended solids concentrations, after incorporation of silt curtain and recommended dredging rate of 55 m³/hour are shown in Table 5-11.

Table 5-11 Suspended Solids Concentration at Distances from Source (Mitigated)

Distance (m)	Predicted Suspended Solids Concentration (mg/L)	Total Suspended Solids Concentration (with background) (mg/L)
50	21.5	35.9
100	10.8	25.2
200	5.4	19.8
250	4.3	18.7
300	3.6	18.0
400	2.7	17.1
500	2.2	16.6
600	1.8	16.2
700	1.5	15.9
800	1.3	15.7
900	1.2	15.6
1000	1.1	15.5

Note: Bold numbers indicate the exceedance of WQO

5.7.14 It is shown that with the recommended mitigation measures, suspended solids concentration at about 200m from the dredging site is predicted to be 5.4mg/L, which complies with the WQO standard of suspended solids. Marine ecological sensitive receivers located to the east and northeast of Tai Lei Island (approximately 250m away from the dredging site) are unlikely to be impacted by elevated suspended solid concentrations.

5.7.15 Within the impact zone (approximately 100m to the west and southwest of the dredging site) some isolated coral patches are identified. The marine ecological survey conducted under this Project revealed that four species of stony coral were found in the concerned area. However, none of them are regarded as rare and uncommon species. The coral coverage in the area was less than 1% and the size of colonies was under 10 cm. The diversity and abundance of the stony corals in the area is very low. The identified stony coral colonies would experience short term and temporary impact from elevated suspended solids concentration. However, historical results in Table 5-3 show that for some years, the concentration of suspended solid could range as high as 45mg/L (SM 10, 1999) and 50.5mg/L (SM 10, 2001). These short term and temporary elevated suspended solids concentrations are within the natural fluctuation range and are considered to be acceptable. Detailed assessment is provided in Section 8.5.

5.7.16 The baseline sediment quality reviewed in Section 5.3 suggested that the contamination level of marine sediment at the vicinity of Peng Chau and Tai Lei Island is considered to be low. Verification of sediment contamination level was performed by sediment sampling and analyses at proposed dredged site for outfall constructions. Category L sediment is found at the proposed dredged area.

Operational Phase

Initial Dilution at Submarine Outfall

5.7.17 The potential water quality impact from Peng Chau STW operation is predicted by near -field modelling. Visjet, a near field model developed by the University of Hong Kong (www.aoe-water.hku.hk/visjet/visjet.htm), based on Lagrangian Jet Model (Jetlag) is used. This model is used to obtain the minimum dilution (initial dilution) achieved when the mixed effluent reaches the water surface or trapping level, the plume radius and the downstream distance of discharged jets.

5.7.18 The near-field modelling for effluent discharge consider the following scenarios at both low and high velocities, during dry and wet seasons:

(1) Planned Average Dry Weather Flow, 1,580 m³/day; and

(2) Planned Peak Dry Weather Flow, 4,740 m³/day

5.7.19 Parameters considered for near field modelling of submarine outfall include the followings::

(a) Outfall length of 100m;

(b) 2 risers (each riser 10m apart), four jets per riser; jet diameter of 100mm;

(c) Seawater Density – Stratified conditions are assumed in wet season. For conservative approach, the density at water surface is assumed to be 14 sigma t units, with 0.5 unit increment every meter down. In dry season, density of 20 sigma t units is used throughout the water column (See Figure 5-3);

(d) Ambient current velocity – Two current velocities representing 10 percentile and 90 percentile of occurrence are assigned. Figures 5-4 and 5-5 show that about 90% of current speed is higher than 0.05m/s and only 10% is higher than 0.4m/s during both wet and dry seasons;

(e) Effluent Density – 998 kg/m³ adopted from SMP 2002; and

(f) Minimum water depth of 8m at discharge point.

5.7.20 Tables 5-12 and 5-13 show the initial dilution obtained at wet and dry seasons, respectively. Typical Visjet output files are shown in Appendix 5A.

5.7.26 For assessment of chronic toxicity, scenario of wet season, average dry weather flow (1,580m³/day) and average ambient current velocity (0.18m/s, 50 percentile of surveyed velocity as shown in Figures 5-4) is adopted. The typical Visjet output file is also shown in Appendix 5A and Table 5-16a below shows the initial dilution factor and TRC concentration.

Table 5-16a TRC Levels at the Edge of Initial Dilution Zone with 1mg/L discharge

	Criteria (mg/L)	Initial Dilution Factor	TRC Concentration at ZID
Acute Toxicity	0.013	76.1	0.013
Chronic Toxicity	0.0075	111.6	0.009

5.7.27 In viewing of the exceedance of chronic toxicity criterion, it is recommended to control the TRC level to 0.8 mg/L in the treated effluent by proper dechlorination procedures. The concentration of TRC at the edge of initial dilution zones, after dechlorination process are shown in Table 5-16b.

Table 5-16b TRC Levels at the Edge of Initial Dilution Zone with 0.8 mg/L discharge

	Criteria (mg/L)	Initial Dilution Factor	TRC Concentration at ZID
Acute Toxicity	0.013	76.1	0.011
Chronic Toxicity	0.0075	111.6	0.007

5.7.28 Results Discussions

Biochemical Oxygen Demand: The increase in BOD at the edge of initial dilution of all modelled scenarios is very low and is not considered to be significant. This suggests that the Dissolved Oxygen (DO) level in the surrounding marine water would not be adversely affected by effluent discharge, due to low biochemical oxygen demand.

Suspended Solids: The percent increase in suspended solids concentration at the edge of initial dilution zones are significantly lower than the WQO standard of 30% (5.9mg/L). The water quality impact of SS from effluent discharge is not significant.

Nitrogen Species: A stringent effluent standard of 10mg/L total nitrogen is required for Peng Chau STW Upgrade to control the discharge of nitrogen species. Such standard can be achieved by incorporating denitrification in the treatment process. The predicted results in Table 5-14 and Table 5-15 show that concentrations of unionised ammonia comply with the WQO criteria at the edge of initial dilution zone for all modelled scenarios.

Although the predicted total inorganic nitrogen concentrations at the edge of initial dilution zones exceed the WQO, the net increase of TIN from the discharge of Peng Chau STW is not considered significant. Exceedance in TIN level is likely due to the high baseline TIN concentration of Southern Water Control Zone. However, in viewing of the past 5 years historical record at water quality monitoring stations SM 10 and SM 11 (Table 5-3), the recorded data ranged from 0.09 to 0.59mg/L (SM 10, 2000). The predicted TIN concentration at the worst scenario (0.411 mg/L, planned peak dry weather flow during wet season at low current velocity) is within this range. The water quality impact from discharge of TIN is considered to be acceptable.

E. coli.: The predicted concentrations of E. coli at the edge of initial dilution zones of all modelled scenarios are significantly less than the WQO of 610 cfu/100mL for the secondary contact recreation subzone, as well as 180 cfu/100mL for bathing beach. The potential water quality impact is minimal.

Total Residual Chlorine: Total residual chlorine (TRC) would be produced during effluent disinfection by chlorination/dechlorination. This parameter is regarded as a toxicant and it should not be present at levels producing significant toxic effect. It is shown in Table 5-16b that with proper dechlorination to a TRC level of 0.8mg/L, criteria at the edge of initial dilution zone would be met.

5.7.29 It is anticipated that in the detailed design stage, other outfall configurations may be explored and adopted. It is recommended that the initial dilution performance of the outfall should be adequate to ensure the compliance of proposed WQC (except TIN where baseline concentration exceeded the WQC).

5.7.30 It can be seen from tables 5-14 to 5-16 that the chronic toxicity criterion of TRC is governing the outfall arrangement design. For recommended 0.8mg/L TRC discharge under wet season, average ambient velocity and ADWF condition, a minimum of 107 initial dilution would require to be achieved during the detailed design of outfall.

Emergency Overflow of Treated Effluent

5.7.31 Raw sewage entering the upgraded Peng Chau STW would receive high level of treatment and would be discharged via the submarine outfall. An emergency overflow outfall is also available to serve as the standby unit when effluent discharge via submarine outfall is not feasible. The proposed emergency overflow pipe is located at the same direction as the submarine outfall. The overflow pipe is approximately 40m long with a pipe diameter of 250 mm. The minimum discharge water depth is 3m.

5.7.32 In case of failure of the duty submarine outfall, treated effluent would be diverted to the emergency overflow outfall. The impact of diverting effluent to the submarine outfall is evaluated. As shown in Table 5-12 and Table 5-13 that the least initial dilution would occur during peak dry weather flow during wet season at low ambient velocity. This discharge scenario is selected for impact evaluation.

5.7.33 As the emergency overflow is submerged under seawater, the initial dilution can be obtained from Visjet. Under stratified wet season with low ambient current velocity, the initial dilution factor is 9.85 (see the output file in Appendix 5B).

5.7.34 Subsequent far-field dilution is estimated using the linear dispersion coefficient version of the Brooks Equation as presented by Grace (1978). The analytical solution has been developed to estimate diffusion of a diluted effluent in a flowing environment. Two scenarios were predicted, one for low diffusion velocity (0.05m/s) and one for high diffusion velocity (0.4m/s). Appendix 5C1 andAppendix 5C2 shows the estimated far-field travel time, distance and effluent dilution factors for low and high velocity, respectively. Summary is provided in Table 5-17.

Table 5-17 Far Field Dilution Along Plume Trajectory

Total Travel Distance (m)	Low Diffusion Velocity		High Diffusion Velocity
Total Travel Distance (m)	Travel Time (hr)	Effluent Dilution	Travel Time (hr)	Effluent Dilution ~~at Low Velocity~~
100	0.55	37.4	0.07	~~37.4~~11.4
200	1.11	67.9	0.14	14.9~~67.9~~
300	1.67	98.4	0.21	18.6~~98.4~~
400	2.22	128.9	0.28	22.4~~128.9~~
500	2.78	159.4	0.35	26.2~~159.4~~
1000	5.55	311.9	0.69	45.3~~311.9~~

5.7.35 It is predicted that at low velocity, effluent dilution of 37.4 can be achieved after 0.55 hour travel time. Similar trend is shown in high velocity, where 26.2 to 45.3 dilution can be achieved in the travelling time of 0.35 to 0.69 hours. However, the total travel distances at high velocity is significantly larger than at low velocity, implying that effluent plume can be washed away quicker than at low velocity. As such, the low velocity scenario is considered as worst case and is further assessed in the following sections.

5.7.36 The predicted total pollutant concentrations at low velocity at selected downstream distances (100m, 300m, 500m and 1000m) are shown in Table 5-18.

Table 5-18 Pollutant Concentrations after Far Field Dispersion (Treated Effluent)

Pollutant Parameters	Discharge (mg/L)	Baseline Conc. (mg/L )	Total Conc. at 100m⁽¹⁾	Total Conc. at 300m⁽¹⁾	Total Conc. at 500m⁽¹⁾	Total Conc. at 1000m⁽¹⁾
BOD	20	1	1.51	1.19	1.12	1.06
SS	30	14.4	14.82	14.56	14.50	14.45
TN	10
NH₄-N	4	0.08	0.185	0.12	0.105	0.093
Unionised Ammonia	0.265	0.004	0.011	0.007	0.006	0.005
TIN	8	0.31	0.516	0.388	0.358	0.335
Total Residual Chlorine	0.8	0	0.021	0.008	0.005	0.003
E. coli	1000 cfu/ 100mL	13 cfu/ 100 mL	40 cfu / 100 mL	24 cfu / 100 mL	20 cfu / 100 mL	17 cfu / 100 mL

Note: (1) Downstream Distance

5.7.37 It is shown that total concentrations of discharged pollutants decrease as the discharged plume migrates further downstream. Although the plume is not dispersed immediately, desirable dilution would be achieved in relatively short time for all the concerned pollutants except TIN, which the baseline concentration has already exceeded the WQO requirement. A longer time is need for the dispersion of TIN. The concentration of TIN within 2 hours of emergency discharge (300m downstream) would be within the natural fluctuation range recorded in the nearby water quality monitoring stations. Moreover, compliance of TRC acute toxicity criterion 0.013mg/L can also be achieved.

5.7.38 The impacts from increase in BOD, SS, ammonium nitrogen, unionised ammonia and E. coli are considered to be acceptable at about 100m downstream. The net increase in total residual chlorine concentration is considered to be acceptable at 300m downstream.

Evaluation of Water Quality from Discharge of Stringent Total Nitrogen Concentration (5mg/L)

5.7.39 If treated effluent comply with alternative effluent standard stipulated in Table 5-9b is discharged, the SS, BOD, TRC and E coli concentration at the ZID after initial dilution would be the same as Table 5-14 and Table 5-15. With the same assumption of nitrogen species composition and their respective portions as 10mg/L Total Nitrogen in Section 5.7.24, the concentrations of nitrogen species from discharge of 5mg/L TN, after initial dilution are shown in Table 5-19.

Table 5-19 Nitrogen Species Concentration at the Edge of Initial Dilution Zone (TN 5mg/L)

Flow Scenario	Pollutant Parameters	Discharge (mg/L)	Baseline Conc. (mg/L )	Wet -Low Velocity	Wet – High Velocity	Dry - Low Velocity	Dry - High Velocity
1,580 m³/d	NH₄-N	2	0.08	0.10	0.092	0.089	0.081
	Unionised Ammonia	0.133	0.004	0.005	0.005	0.005	0.004
	TIN	4	0.31	0.348	0.333	0.327	0.312
4,740 m³/d	NH₄-N	2	0.08	0.105	0.093	0.094	0.082
	Unionised Ammonia	0.133	0.004	0.006	0.005	0.005	0.004
	TIN	4	0.31	0.358	0.335	0.336	0.314

5.7.40 The predicted nitrogen species concentrations show that compliance with WQO on unionised ammonia is achieved. Exceedance of TIN WQO is also found due to elevated background TIN concentration in Southern Water Control Zone, although net increases in TIN at the edge of initial dilution zones are considered insignificant.

5.7.41 It is anticipated that emergency discharge of treated effluent with 5mg/L total nitrogen would exhibit similar impact as Table 5-18. The unionised ammonia would comply with WQO but exceedance in TIN would be anticipated.

5.7.42 The difference between water quality impact from 10 mg/L and 5 mg/L Total Nitrogen discharge is not significant.

Emergency Overflow of Untreated Effluent

5.7.43 In the case of failure of STW treatment units, raw sewage may be diverted to submarine outfall or emergency overflow outfall for discharge. Although these cases would be extremely rare with the incorporation of standby STW units, the worst case of discharge via emergency overflow outfall is evaluated.

5.7.44 Scenario of peak dry weather flow discharge and loads (see Table 5-9a) is evaluated as the worst case scenarioPeak dry weather flow with raw sewage load concentration is evaluated as worst case scenario. The dilution obtained from far field dispersion is the same as the value presented in Table 5-17. The predicted total pollutant concentration at 100m, 300m, 500m and 1000m downstream are provided in Table 5-20.

Table 5-20 Pollutant Concentrations After Far Field Dispersion (Untreated Discharge)

Pollutants	Discharge Conc. (mg/L)	Baseline Conc. (mg/L)	Total Conc. at 100m	Total Conc. at 300m	Total Conc. at 500m	Total Conc. at 1000m
BOD	202	1.0	6.37	3.04	2.26	1.64
SS	182	14.4	18.9	16.1	15.5	14.9
TKN	39	0.225	1.262	0.619	0.468	0.349
Ammonium	23	0.08	0.69	0.31	0.22	0.15
E.Coli	1.9x 10⁷cfu /100 mL	13 cfu /100 mL	5.1 x 10⁵cfu /100 mL	1.9 x 10⁵cfu /100 mL	1.2 x 10⁵cfu /100 mL	8.1 x 10⁴cfu /100 mL
Unionised* Ammonia	1.52	0.004	0.045	0.019	0.014	0.009

Note: * assuming 6.625% of ammonium become unionised ammonia.

5.7.45 As the pollutant loading of untreated effluent is significantly larger than the treated effluent, longer dispersion time is required to achieve an adequate dilution. It is shown in Table 5-20 that the unionised ammonia concentration violates the WQO at 100m downstream. Compliance of unionised ammonia concentration can be achieved at about 300m downstream.

5.7.46 The predicted results show that concentration of E. coli would not comply with the WQO criteria of 610cfu/100mL for secondary contact recreation zone and 180 cfu/100 ml for bathing beach after several hours of dispersion. However, as no decay coefficient was incorporated into the Brooks Equation estimation, the predicted results for E. coli are conservative. It is expected that bacteria die off would occur in the natural marine environment.

5.7.47 Discharge parameters BOD, SS, TKN, unionised ammonia and ammonium after far field dispersion of emergency overflow discharge would be within the annual fluctuation ranges after 5.55 hours of dispersion (1000m downstream) and the potential water quality impacts are considered acceptable.

5.7.48 It is recommended that emergency overflow of untreated sewage should be avoided as far as practicable. Proper mitigation measures can be incorporated into the design of treatment process. Detailed mitigation measures are elaborated in later sections.

5.8 Mitigation Measures

Construction Phase

Upgrade of STW

5.8.1 The practices outlined in ProPECC PN 1/94 Construction Site Drainage are recommended to be adopted to minimise the potential water quality impacts from construction site runoff and various construction activities. Perimeter channels are to be installed in the works areas to intercept runoff at site boundary prior to the commencement of any earthwork. Intercepting channels are to be provided to prevent storm runoff from washing across exposed soil surfaces. Drainage channels are also required to convey site runoff to sand/silt traps and oil interceptors. Regular cleaning and maintenance are to be provided to ensure the normal operation of these facilities throughout the construction period.

5.8.2 It is anticipated that the wastewater generated from the works areas would be in small quantity as the construction scale is not considered large. The construction programme should be properly planned to minimise soil excavation in rainy seasons. This would prevent soil erosion from the exposed soil surfaces. Any exposed soil surfaces should also be properly protected to minimise dust emission. Exposed stockpiles should be covered with tarpaulin or impervious sheets at all times.

5.8.3 Good site practices should be adopted to clean the rubbish and litter on the construction sites so as to prevent the rubbish and litter from dropping into the nearby marine environment. It is recommended to clean the construction sites on a regular basis.

Sewage from Workforce

5.8.4 The presence of workforce for the construction generates domestic sewage. It is anticipated that during the upgrading construction, domestic sewage will be collected and discharged to the STW for proper treatment. The amount of sewage generated by the workforce is not significant as compared with the regular treatment capacity of the STW.

Submarine Outfall and Emergency Overflow Outfall

5.8.5 In order to alleviate potential water quality impacts from the construction of the Project, the following mitigation measures should be implemented during the construction of the submarine outfall and emergency overflow outfall:

o Dredging should be undertaken using closed grab dredgers with a total production rate of 55 m³/hr;

o Deployment of ~~2-layer~~ silt curtains with a minimum solids reduction efficiency of 75% or higherfrom the dredging area while dredging works are in progress;

o All vessels should be sized such that adequate clearance (i.e. minimum clearance of 0.6m) is maintained between vessels and the sea bed at all states of the tide to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash;

o All pipe leakage should be repaired promptly and plant shall not be operated with leakage pipes;

o Excess material should be cleaned from the decks and exposed fittings of barges before the vessel is moved;

o Adequate freeboard should be maintained on barges to ensure that decks are not washed by wave action;

o All barges should be fitted with tight fitting seals to their bottom openings to prevent leakage of material;

o Loading of barges and hoppers should be controlled to prevent splashing of dredging material to the surrounding water, and barges and hoppers should not be filled to a level which would cause the overflow of materials or sediment laden water during loading or transportation; and

o The decks of all vessels should be tidy and free of oil or other substances that might be accidentally or otherwise ~~wached~~ washed overboard.

Operational Phase

Emergency Overflow

5.8.6 Discharge of treated effluent will be diverted to the emergency overflow pipe if the case of damage of submarine outfall pipe. Under normal circumstance, each process unit will be backed up with a standby unit. The standby generator will ensure the continuous electricity supply for the STW. In-line and/or off-line equalization tanks of 1000m³ will be constructed to provide the buffer zone for influent and/or effluent storage. From the water quality point of view, the discharge of treated effluent from the emergency overflow pipe will likely meet the minimum effluent standard for this project. As such, the emergency overflow pipe serves as a standby unit for the submarine outfall pipe from this perspective.

5.8.7 For conditions where damages occurred in any of the STW unit, standby unit will be operated and the designed treatment capability would be restored immediately to ensure that water quality of the effluent can meet the discharge requirement. Hence, no observable impact to both the submarine outfall and the emergency overflow pipes is anticipated.

5.8.8 Under abnormal conditions where any treatment units fail to achieve the targeted effluent quality, the impact to the receiving water body can be alleviated through the dilution after discharging from the submarine outfall or emergency overflow pipes. Notwithstanding that the chance of such failure is unlikely to occur, the impact is considered acceptable as the effect will only happen in short-run.

5.8.9 In the case of STW overflow, raw sewage may also be diverted to emergency overflow outfall although discharge through submarine outfall is more preferable for dilution and dispersion. In an extreme situation where no electricity supply is available (including the failure of the standby generator, the impact from the untreated raw sewage can also be alleviated through the initial dilution after discharging from submarine outfall for emergency overflow pipes. Although the level of E coli. may be over the acceptable range, this approach is considered as the best option in view of the low chance of discharging the raw sewage directly to the receiving body in a long run.

5.8.10 Based on the above, it is extremely unlikely that major duty and standby units of the STW, the submarine outfall pipe and electricity supply would have problem simultaneously. Besides, it is very rare that the event will last for a long time. Hence, the probability of diverting the discharge via emergency overflow is quite low. Nonetheless, any effluent discharge with substandard water quality should notify EPD and DSD at this extreme case.

5.9 Cumulative Impacts

Construction Phase

5.9.1 Construction water quality impact from the reclamation works of the Peng Chau Helipad Project would have potential impacts as the dredge and fill activities are carried out within the 1 km of the water quality assessment area of Peng Chau STW Upgrade. Given the relatively small dredged area, it is predicted that the construction impact is localised within the assessment area.

5.9.2 The reclamation period of Peng Chau Helipad is expected to be about 7 months and is scheduled from May 2005 to December 2005 and the assessment area is 2km. The preliminary construction programme (Figure 2-6) shows that the construction of submarine and emergency overflow outfall is scheduled to start in May 2006. It is recommended to specify the outfall construction marine works after May 2006 in the contract document to avoid potential cumulative impact from reclamation of Peng Chau Helipad. No cumulative impact would then be anticipated.

Operational Phase

5.9.3 No water quality impact is identified during the operation of Peng Chau Helipad. No operational water quality cumulative impact is anticipated.

5.9.4 If Peng Chau Typhoon Shelter were to be implemented, the breakwater structure to the south of Tai Lei Island would further reduce the water movement. This may lead to decrease in initial dilution of outfall discharge. However, as the zones of initial dilution of all modelled scenarios are small and localised, the dilution characteristics are likely to be affected but not significant. The impact to the water quality would not be unacceptable.

5.10 Residual Water Quality Impacts

Construction Phase

5.10.1 The residual water quality impact, with the implementation of construction mitigation measures as recommended, would not be unacceptable to the small coral colonies found in the impact area, given the exceedance of suspended solids is minor (within the annual fluctuation range) and temporary.

Operational Phase

5.10.2 Residual water quality impacts is expected to be acceptable under the normal operation conditions. The design of the submarine outfall is adequate to cater for a range of daily flow from 1,580 m³ to 4,740 m³. TIN concentrations exceeding the WQO limit are observed for scenarios under both wet season and dry season. These exceedances are contributed by the elevated baseline TIN concentration in the Southern WCZ. The net increases in TIN due to effluent discharge are not significant.

5.10.3 Discharge of untreated sewage through emergency overflow would have water quality impact to the surrounding marine environment. This discharge scenario would happen during the failure of both the treatment works and submarine outfall. The water quality impact is mainly from bacteria E. coli of untreated sewage. Given the sensitivity of surrounding water of Peng Chau and Tai Lei Island as secondary contact reaction zone, as well as a non-gazetted Tung Wan Beach within the assessment area, presence of E. coli exceeding the WQO limit is undesirable. Although failure of both STW components would be rare, mitigation measures as recommended in Section 5.8 (from 5.8.6 to 5.8.10) should be implemented to minimise the occurrence.

5.11 Environmental Monitoring and Audit

5.11.1 Environmental Monitoring and Audit for water quality would be required for construction of submarine and emergency overflow outfalls. An EM&A programme including monitoring of pre- and post dredging water quality would be required to ensure the implementation of the recommended water quality mitigation measures during the construction period. As the predicted TIN during operational phase would exceed the WQO limit, it is recommended to carry out water quality monitoring for TIN during the initial operational stage. Details of the EM&A procedures are presented in a separate EM&A Manual.

5.12 conclusions and recommendations

Construction Phase

5.12.1 Dredging activities for the construction of submarine and emergency overflow outfalls would elevate the suspended solids concentration in the marine environment. Assessment of sediment dredging has been carried out by predicting the increase of sediment concentration due to fine sediment lost to suspension.

5.12.2 High level of sediments which exceed the WQO criteria are predicted in the vicinity of the study area where marine ecological sensitive receivers coral are found. Mitigation measures are proposed for dredging activities. It is recommended that closed grab dredger and silt curtains are to be used and reduction in dredging rate is to be implemented. With the implementation of proposed mitigation measures, slight exceedance of WQO criteria is predicted within 200m of the dredged alignment. The exceedance would be temporary. The elevated level is within the natural fluctuation range and the construction water quality impact is considered to be acceptable.

Operational Phase

Effluent Discharge Via Submarine Outfall

5.12.3 Water quality impacts from the operation of Peng Chau STW upgraded have been assessed. Assessments of the near field dispersion from discharge of treated effluent have been carried out by using Visjet model for both dry and wet seasons. The modelling results show that adequate initial dilutions can be achieved for all discharge scenarios (planned average and peak average scenarios) at both seasons. The total concentration of SS, unionised ammonia and E.coli would comply with WQO standards after initial dilution. The increase in BOD concentrations would not elevate the background concentration significantly.

5.12.4 Although no WQ standard is available for total residual chlorine (TRC- as a toxicant), USPEA criteria of 0.013mg/L and 0.0075mg/L are adopted at the edge of initial dilution zone for acute and chronic toxicity, respectively. The predicted results show that with TRC reduction to 0.8mg/L, the proposed criteria can be met and water quality impacts are not considered to be significant.

5.12.5 Total concentrations of TIN are predicted to exceed the WQO standard, owing to its high baseline concentration. The net increases in TIN due to effluent discharge are not significant and the dilution provided by the near field effect allows the TIN concentration to fall within the natural variation/fluctuation range of a water quality monitoring station SM 10. The water quality impacts are thus considered acceptable.

5.12.6 It is expected that operation of the upgraded Peng Chau STW would be beneficial to the surrounding receiving water body. The current Peng Chau STW provides secondary treatment to sewage collected from residential developments at northern Peng Chau. Sewage from some village houses is still being discharged untreated. Prior to the operation of the upgraded STW, the Peng Chau Package K project would connect and divert the sewage from the majority Peng Chau household to the STW. Less discharge of untreated sewage would be anticipated. With the incorporation of denitrification process into the upgraded Peng Chau STW, the discharge of total inorganic nitrogen into the receiving water body would be substantially reduced. Long term improvement of water quality is anticipated.

Emergency Overflow

5.12.7 Discharges of treated effluent via emergency overflow outfall in the case of damage or blockage of submarine outfall have been assessed. Visjet model was used for initial dilution prediction and the subsequent far field dilution was determined by Brooks Equation. The predicted results show that adequate dilution can be achieved within a few hours after initial dilution. The predicted concentration of SS, unionised ammonia and E.coli would comply with WQO standard after far field dispersion. Concentration of total residual chlorine and BOD would be insignificant. TIN remains as an exceeding parameter due to high baseline concentration.

5.12.8 In the case of failure of both STW and submarine outfall, untreated sewage would have to be discharged via the emergency overflow outfall. Water quality impact from peak dry weather flow scenario has also been assessed using the same methodology. The discharged effluent would have significantly higher pollution load than the treated effluent and a longer time is required for the effluent plume to achieve adequate far field dilutions. At the 1km water quality impact assessment zone, concentrations of SS and unionised ammonia would comply with WQO standards. Both TKN and BOD concentrations would be in the ranges of natural fluctuation of nearby water quality monitoring stations. The concentration of E.coli, however, would still exceed the WQO criteria for secondary contact recreation zone. Mitigation measures are recommended to minimise the chance of emergency overflow of untreated effluent.

5.12.9 It is extremely unlikely that major duty and standby units of the STW, the submarine outfall pipe and electricity supply would have problem simultaneously. Besides, it is very rare that the event will last for a long time. Hence, the probability of diverting the discharge via emergency overflow is quite low. Nonetheless, any effluent discharge with substandard water quality should notify EPD and DSD at this extreme case.

Scenarios	Initial Dilution Low Velocity (0.05m/s)	Downstream Distance (m) at Low Velocity	Estimated Zone of Initial Dilution (m²)*	Initial Dilution High Velocity (0.4m/s)	Downstream Distance (m) at High Velocity	Estimated Zone of Initial Dilution (m²)
Average Dry Weather Flow (1,580 m³/day)	98.0	4	160	157.3	27	1,080
Peak Dry Weather Flow (4,740 m³/day)	76.1	5	200	148.7	27	1,080

Scenarios	Initial Dilution Low Velocity (0.05m/s)	Downstream Distance (m) at Low Velocity	Estimated Zone of Initial Dilution (m²)*	Initial Dilution High Velocity (0.4m/s)	Downstream Distance (m) at High Velocity	Estimated Zone of Initial Dilution (m²)
Average Dry Weather Flow (1,580 m³/day)	219.2	5	200	2291.6	156	6,240
Peak Dry Weather Flow (4,740 m³/day)	141.0	6	240	903.4	82	3,280

Pollutant Parameters	WQC or USEPA Criteria (mg/L)	Discharge (mg/L)	Baseline Conc. (mg/L )	Average Flow
Pollutant Parameters	WQC or USEPA Criteria (mg/L)	Discharge (mg/L)	Baseline Conc. (mg/L )	Wet -Low Velocity	Wet – High Velocity	Dry - Low Velocity	Dry - High Velocity
BOD	--	20	1	1.19	1.12	1.09	1.01
SS	Increase by < 30%	30	14.4	14.56	14.50	14.47	14.41
TN	--	10	--
NH₄-N	--	4	0.08	0.12	0.105	0.098	0.082
Unionised Ammonia	0.021 mg/L	0.265	0.004	0.007	0.006	0.005	0.004
TIN	0.1 mg/L	8	0.31	0.388	0.359	0.345	0.313
E. coli	610 cfu/100ml* 180 cfu /100ml **	1000 cfu/100mL	13 /100 mL	23/100mL	19/100mL	17/100mL	13/100mL

5 Water Quality

5.1 Introduction

5.1.2 The near field model Visjet is employed to predict the initial dilution of sewage plume during the operational phase of Peng Chau STW upgrade. No far-field hydrodynamic modelling is required as the effluent discharged from the proposed STW upgrade would be small.

5.2 Relevant Legislation, Policies, Plans, Standards and Criteria

Water Pollution Control Ordinance

Technical Memorandum for Effluents Discharged into drainage and Sewerage Systems Inland and Coastal Waters

5.3 EXISTING MARINE environment

Surrounding Marine Environment and Conditions

5.3.2 The marine conditions of Peng Chau, as described in the SMP 2002 are:

5.3.3 The Peng Chau islands and Tai Lei are located within the beneficial use of secondary contact recreation area (Figure 5-1).

Baseline Conditions

5.3.5 EPD also undertakes sediment analyses in the Southern WCZ. The closest sediment monitoring station to Peng Chau and Tai Lei Island is SS5 (Figure 5-1). The sediment quality from 1997 to 2001 has been summarised and presented in Marine Water Quality in Hong Kong in 2001 (EPD, 2002c).

5.3.7 In addition, sediment sampling and analysis was carried out to the southwest of Tai Lei Island, under this Project. Detail findings of this sediment quality results are presented in Section 6.

5.3.8 The EPD water quality and sediment quality monitoring data, as well as the survey results presented in Maunsell 1997 reports are summarised and discussed in the following sections.

Marine Water Quality

5.3.9 Table 5-3 presents the summary statistics of key water quality parameters from the EPD monitoring stations SM 10 and SM 11.

5.3.10 Review of historical summary statistics indicated the following:

5.3.11 In summary, EPD data indicate that marine water surrounding Peng Chau and Tai Lei generally exhibits good quality except TIN, for which the baseline level has already exceeded the WQO level. The elevated baseline TIN may be due to the discharge from Pearl River Estuary.

5.3.12 Monitoring results of selected parameters of marine water quality from the Manusell 1997 Studies are given in Tables 5-4 and 5-5.

Stratification Conditions

5.3.14 Pearl River Estuary discharge also affects the salinity of Hong Kong marine environment. Although fresh water enters Hong Kong waters all year around, relatively large volume flushes in during the wet season, causing the upper seawater to be less dense than the bottom seawater.

Current Velocity and Direction

Marine Sediment Quality

5.3.23 According to the findings in the sediment sampling and analyses conducted under the Project, the sediment quality to the southwest of Tai Lei Island was determined to be Category L.

5.4 Marine Water Sensitive Receivers

5.5 Design capacity and effluent requirement of Peng Chau STW upgrade

5.6 Impact Identification

Construction Phase

Upgrade of Peng Chau Sewage Treatment Works

5.6.1 The proposed project involves the following construction works:

5.6.2 The potential construction water quality impacts can be categorised into land-based impacts and marine based impacts.

5.6.5 The anticipated impact from the construction of terrestrial components would be insignificant. Nevertheless, for good site practice and appropriate mitigation measures are recommended to be implemented during the construction of new facilities and demolition of old facilities.

Submarine Outfall and Emergency Overflow Outfall

Operational Phase

5.6.11 The positive impact, however, can not be quantified as there is no available information on the untreated discharge. The environmental impacts identified during operational phase are the chemical species of concern which exists in the sewage effluent.

5.6.21 Given the small flow of effluent discharge and the provision of submarine outfall for effluent dispersion, the organic-CBP in the treated effluent should have insignificant impact to both the ecology and human health.

Total Residual Chlorine

5.6.25 The implementation of Peng Chau Typhoon Shelter would further reduce the water movement in the area. Potential cumulative impacts are addressed in the later sections.

5.7 IMPACT Assessment

Construction Phase

5.7.1 As described in the previous sections, the water quality impact from construction and demolition of land base components of Peng Chau STW upgrade is not significant. Nevertheless, mitigation measures are recommended to be implemented for good site practice.

5.7.3 The construction of submarine and emergency outfall would involve dredging and disposal of approximately 22,000 m3 of marine mud. The cross section of the dredged area is provided in Figure 5-8.

5.7.6 The tidal excursion is thus calculated to be 8.25km and hence this approach may be considered appropriate because of the low rate of dredging and thus the expected limited extent of the plumes, which will certainly be within the tidal excursion.

5.7.7 The formula for estimating the concentration of suspended solids (SS) at a certain distance from the source is:

5.7.9 For the impact assessment of dredging activities in the surrounding water of Tai Lei Island, the more conservative sediment loss rate of 25kg/m3 is adopted as there are two barging piers on the Island. It is expected the debris on the seabed may hinder the dredging operation.

Operational Phase

5.7.18 The near-field modelling for effluent discharge consider the following scenarios at both low and high velocities, during dry and wet seasons:

5.7.19 Parameters considered for near field modelling of submarine outfall include the followings::

5.7.20 Tables 5-12 and 5-13 show the initial dilution obtained at wet and dry seasons, respectively. Typical Visjet output files are shown in Appendix 5A.

5.7.23 Based on the assumption that 6.625% of ammonium becomes unionised (average salinity of 30 psu, temperature 24oC and pH 8.2 in WQ monitoring station SM 10 and SM 11), the unionised ammonia concentration from the effluent discharge would be 0.265 mg/L.

5.7.24 Pollutant concentrations at the edge of mixing zones (after initial dilution) of planned average flows and planned peak flows are shown in Table 5-14 and Table 5-15, respectively.

5.7.28 Results Discussions

Emergency Overflow of Treated Effluent

5.7.33 As the emergency overflow is submerged under seawater, the initial dilution can be obtained from Visjet. Under stratified wet season with low ambient current velocity, the initial dilution factor is 9.85 (see the output file in Appendix 5B).

5.7.36 The predicted total pollutant concentrations at low velocity at selected downstream distances (100m, 300m, 500m and 1000m) are shown in Table 5-18.

5.7.38 The impacts from increase in BOD, SS, ammonium nitrogen, unionised ammonia and E. coli are considered to be acceptable at about 100m downstream. The net increase in total residual chlorine concentration is considered to be acceptable at 300m downstream.

Evaluation of Water Quality from Discharge of Stringent Total Nitrogen Concentration (5mg/L)

5.7.41 It is anticipated that emergency discharge of treated effluent with 5mg/L total nitrogen would exhibit similar impact as Table 5-18. The unionised ammonia would comply with WQO but exceedance in TIN would be anticipated.

5.7.42 The difference between water quality impact from 10 mg/L and 5 mg/L Total Nitrogen discharge is not significant.

Table 5-20 Pollutant Concentrations After Far Field Dispersion (Untreated Discharge)

5.7.47 Discharge parameters BOD, SS, TKN, unionised ammonia and ammonium after far field dispersion of emergency overflow discharge would be within the annual fluctuation ranges after 5.55 hours of dispersion (1000m downstream) and the potential water quality impacts are considered acceptable.

5.7.48 It is recommended that emergency overflow of untreated sewage should be avoided as far as practicable. Proper mitigation measures can be incorporated into the design of treatment process. Detailed mitigation measures are elaborated in later sections.

5.8 Mitigation Measures

Construction Phase

5.8.3 Good site practices should be adopted to clean the rubbish and litter on the construction sites so as to prevent the rubbish and litter from dropping into the nearby marine environment. It is recommended to clean the construction sites on a regular basis.

Submarine Outfall and Emergency Overflow Outfall

5.8.5 In order to alleviate potential water quality impacts from the construction of the Project, the following mitigation measures should be implemented during the construction of the submarine outfall and emergency overflow outfall:

Operational Phase

Emergency Overflow

5.9 Cumulative Impacts

5.9.3 No water quality impact is identified during the operation of Peng Chau Helipad. No operational water quality cumulative impact is anticipated.

5.7.3 The construction of submarine and emergency outfall would involve dredging and disposal of approximately 22,000 m³ of marine mud. The cross section of the dredged area is provided in Figure 5-8.

5.7.9 For the impact assessment of dredging activities in the surrounding water of Tai Lei Island, the more conservative sediment loss rate of 25kg/m³ is adopted as there are two barging piers on the Island. It is expected the debris on the seabed may hinder the dredging operation.

5.7.23 Based on the assumption that 6.625% of ammonium becomes unionised (average salinity of 30 psu, temperature 24^oC and pH 8.2 in WQ monitoring station SM 10 and SM 11), the unionised ammonia concentration from the effluent discharge would be 0.265 mg/L.